1. Field of the Invention
The present invention refers to a hermetic refrigeration motor compressor unit, in particular for use in household-type refrigeration appliances.
2. Description of the Related Art
The actual refrigeration compressor (which is usually of the reciprocating or rotary type) and the related electric driving motor are generally known to have a common shaft and to be enclosed in a sealed casing. Owing to the resulting impossibility for any kind of maintenance to be carried out subsequently, it is imperative for an effective, continuous and reliable lubrication of the moving parts of such a hermetically sealed unit to be assured throughout the operating life span, which may amount to as many as more than 15 years, of the refrigeration appliance in which such a unit is mounted.
To this purpose, the hermetic motor compressor unit comprises means that are adapted to ensure, during the operating periods thereof. an adequate flow of the oil that is filled in an appropriate quantity in the sealed casing at the end of the compressor manufacturing cycle, in such a manner as to enable it to not only lubricate said moving parts, but also prevent them from overheating.
It is a generally known practice for the above mentioned means to be provided in the form of a downward extension of the shaft, that is longitudinally bored and shaped so as to be capable of drawing in the oil from the bottom of the sealed casing and diffusing it, substantially by centrifugal effect, into the parts to be lubricated. Such a construction is usually effective in the case of traditional hermetic motor compressor units that are equipped with a single-phase two-pole induction motor operating at 2700 to 3000 rpm.
However, hermetic motor compressor units are increasingly being used since a few years now, which are equipped with electric direct-current brushless motors that are capable of operating at adjustable speeds varying from a maximum that may reach as many as 4000 rpm to a minimum that may on the contrary be as low as 1000 rpm or even less, to the purpose of optinizing the performance of the related refrigeration appliance, particularly from an energy usage point of view.
Hermetic motor compressor units are also known which are equipped with a four-pole induction motor operating at a speed of only 1500 rpm so that, for a same refrigeration capacity or output, a greater displacement of the compressor is needed.
In both above cited cases, the traditional lubrication and cooling means as the afore mentioned ones are largely inadequate owing to the fact that the low rotational speeds of the shaft do not ensure any adequate delivery head and flow rate of the oil sucked in from the bottom of the sealed casing, or a thorough diffusion of the same oil to all parts that need to be lubricated.
In a hermetic motor compressor unit in which the electric driving motor is arranged below the compressor, it has been proposed that a worm-screw member be inserted with a slight radial gap in the hollow shaft starting from the lower end thereof (see document WO 96/29516). The relative movement between the shaft and the worm-screw member, which extends along the entire electric driving motor and is retained by a support being fixed to the stator of the motor, ensures the desired oil flow. This solution has however a drawback in that is the helical insert is completely contained inside the shaft and is affected by the heat transmitted by the motor. As a result, mutual expansions tend to take place during the operation of the motor compressor unit, which cause the radial gap between the insert and the shaft to be altered, under corresponding variations in the lubrication performance.
It has been also proposed that a pumping chamber with a worm-screw member attached protrudingly to the lower end portion of the shaft and a therewith associated stationary sleeve (see documents WO 93/22557 and EP-A-0 728 946) be provided in the motor compressor unit. This kind of solution causes a further radial gap to be created between the stationary sleeve and the inner surface of the rotating shaft, so that leakages unavoidably take place owing to oil seeping through said gap, with the result of a reduced flow rate of the oil being pumped. Furthermore, a solution of this kind proves quite difficult and expensive to implement, since care shall duly be taken to ensure, between the two component parts of the pumping chamber, not only the afore mentioned circumferential gap, but also a gap on the horizontal plane between the sleeve and the lower end portion of the shaft in view of preventing said parts from undergoing a weardown effect brought about by friction during operation.
Patent Abstracts of Japan vol. 009, no. 275 (M-426), Nov. 2, 1985 (1985-11-02) and JP 60 119389 A (Toshiba KK), Jun. 26, 1985 (1985-06-26) disclose a lubricating system also comprised of a fixed helically grooved (fluted) element, which can also be considered to be connected elastically with the stator. However, in said documents a separate rotating sleeve fully outside the rotor is not provided, but rather a rotating hollow motor shaft is utilized as part of the oil suction means. Therefore, the oil suction means are not fully outside the rotor shaft, because the fixed element is inserted within the rotor shaft. Such a solution is affected by serious heat transfer problems.
It therefore is a main purpose of the present invention to provide, through the use of simple, reliable and low-cost means, an improved hermetic motor compressor unit that is capable of ensuring an effective oil flow even at operating speeds thereof that may be as low as approx. 800 rpm.
As this will be better explained further on, this and further aims are reached in a hermetic motor compressor unit provided with an oil suction arrangement featuring the characteristics as recited in the appended claims.